Heat-sealable, peelable films have long been used in the packaging of foodstuffs and beverages in order to keep goods sealed and fresh in a container prior to the end-use by the consumer. Typically, heat-sealable peelable films are multi-layer films, in which a substrate layer provides the requisite strength, and one or more additional layer(s) provide the heat-sealable functionality. Examples of such so-called “sealant films” include composite films wherein the substrate layer comprises polyester (typically polyethylene terephthalate (PET)) onto which is extrusion-coated a heat-sealable layer. A problem with prior art multi-layer films is that the inter-layer adhesion of the composite film can be insufficient for the sealant film to be readily peeled off and removed from the container by the consumer. Thus, in the case of some prior art sealant films, all or part of the heat-seal layer can sometimes remain on the container when peeling the sealant film, which is typically referred to as adhesive failure. Accordingly, it would be desirable to increase the inter-layer adhesion and provide a “clean peel”. Previous attempts to increase the inter-layer adhesion of composite films have involved the use of a “tie-layer” between the top and bottom layers of the multi-layer film. A tie-layer comprises a layer of adhesive resin which is coextruded with the heat-sealable layer onto the substrate layer using coextrusion-coating. However, even with the presence of conventional tie-layers, it has been found that the inter-layer adhesion between the substrate and the tie-layer and/or between the tie-layer and the heat-sealable layer decreases as the thicknesses of the tie-layer and heat-sealable layer decrease. For this reason, the prior art films typically required that the total thickness of the tie-layer and heat-sealable layer was relatively high, and such films are consequently relatively costly to manufacture because of the amount of polymer material used. The thickness of the tie-layer itself is typically at least 4 μm. The use of coextrusion-coating to apply the sealable layer and tie-layer also makes the manufacture of the film more complex and more expensive. It would be desirable to provide a composite heat-sealable film comprising a relatively lower amount of heat-sealable material without detriment to the heat-seal characteristics of the film. Such a film would be of general use in packaging applications in which a heat-sealable peelable film is required.
One such packaging application is the cap-liner. A cap-liner is a device sealed on the top of a bottle in order to preserve, and provide security to, the liquid stored inside the bottle and to avoid leaking. The cap-liner is peeled off the bottle prior to emptying the liquid stored inside. Various materials have been proposed as layers in cap-liners, including polyolefin and polyester film, and the cap-liners known in the art typically comprise multi-layer laminate films. The bottom part (i.e. the part in closest proximity to the bottle to be sealed) typically comprises a sealant film comprising a layer of heat-sealable material which is heat-sealed to the rim of the bottle. The upper part (i.e. the part most remote from the bottle to be sealed) of the cap-liner structure typically comprises a white film and a device to help the consumer remove the cap-liner from the bottle, for instance a layer of circular film which is folded in half and adhered only to one half of the uppermost surface area of the cap-liner. The upper part of the cap-liner is adhered to the sealant film, typically with an intermediate layer of aluminium foil. For the reasons described above, a heat-sealable peelable film which exhibited improved inter-layer adhesion but which used less heat-sealable polymeric material and which was therefore thinner would be of benefit in this application.
A further problem with conventional packaging materials is the range of substrates to which a given packaging material can be heat-sealed. Thus, with regard to cap-liners, the known cap-liners cannot normally be used over a range of bottle types. Typically, bottles and other substrates are made from polymeric materials such as polypropylene (PP), polystyrene (PS), polyethylene (PE) and polyester (such as polyethylene terephthalate (PET)) as well as glass. It would be desirable to provide a packaging, including cap-liners, which would adhere to a larger variety of substrate materials, both polar and non-polar materials, particularly polyester (particularly PET), polyolefin and glass, and particularly polyester (particularly PET) and glass. In the case of cap-liners, it would be desirable to provide a cap-liner which would adhere to a larger variety of bottles, particularly polyester (particularly PET), polyolefin (particularly PE, PP and PS) and glass, and particularly polyester (particularly PET) and glass.
It is an object of this invention to overcome one or more of the afore-mentioned problems.